590 A. R. Martin and V. M. F. da Silva
these waterways as a source of food and/or for transport, or whose activities pollute the water with sewage, noise and chemical or physical waste, is increasing. At the current rate, the human population of the state of Amazonas, which covers 22% of the Amazon basin, is expected to dou- ble every 79 years (IBGE, 2019). It is therefore likely that the number of gillnets, in which dolphins often drown, will in- crease, and that the density of fish, including those species that the dolphins feed on, will decrease. Although newdams are not planned for the area around Mamirauá, 227 new hydro-electric dams are planned for the Amazon basin as a whole (Castello &Macedo, 2016; Forsberg et al., 2017), al- most all of which can be expected to have a negative impact on river dolphins, directly or indirectly (Araújo & Wang, 2015). As a consequence of these changes, annual survival rates of dolphins are likely to decrease. This model predicts that the impact of reducing annual survival by just 1%brings forward the point at which 95% of model iterations predict extinction to year 141 of the model run. If survival is reduced by 3% this happens in year 110 even if no breeding depres- sion at low population levels is assumed. The equivalent figures with breeding depression are year 108 and 88, respectively. Predicting when extinction is likely to occur is dependent
upon a number of assumptions that cannot be tested, but the models leave no reasonable doubt that extinction will occur unless key population parameters improve. Reproductive characteristics are unlikely to be influenced by human-induced change at present, but survival rates can be. Although many different anthropogenic factors negatively affect boto populations, fishing practices cause the greatest damage. Botos are drowned in fishing nets, especially monofilament gillnets. Despite laws prohibiting it, they are also deliberately killed, especially for use as fish bait (da Silva et al., 2011; Iriarte & Marmontel, 2013a,b; Brum et al., 2015). Enforcement of existing fishery laws throughout the species’ range would increase its chances of survival. In addition, measures to reduce mortality in gill- nets are necessary, particularly during the calving season. There is evidence to suggest that a large proportion of calves die in the first few months of life (Martin & da Silva, 2018), and drowning because of entanglement in gillnets is probably a major contributor to this mortality. The peak of calving and early calf care in September– November coincides with low water levels, when most botos are confined to edges of the main rivers and when high densities of large-mesh gillnets are set to catch fish there (Martin & da Silva, 2018). Neonate botos are neither sufficiently experienced to avoid gillnets, nor strong enough to escape them once entangled. A single encounter with a gillnet during the first 3 months of life would thus likely be fatal. Another important threat to the long-term survival of boto populations is the segmentation of rivers by dams
across their range. These structures have profoundly damaging impacts on the ecological functioning of rivers (Lees et al., 2016; Forsberg et al., 2017), and harm dolphins by disrupting fish migration and preventing genetic exchange between dolphins either side of the dam. The scale of dam building planned for the Amazon basin (Forsberg et al., 2017), if realized, is likely to increase therateofdeclineofbotos andadvance thedateof their extinction. Although the present population trajectory points
towards extinction, there is still time to reverse the decline. Botos are still in sufficient abundance for reproduction to occur normally, and as yet relatively few rivers are dammed to the extent that animal movement is constrained, so gen- etic interchange is continuing. But the results of this study demonstrate that effective actionmust soon be taken by the range states of the Amazon river dolphin if this iconic ani- mal is to survive. One important first step was taken by the government of Brazil: the banning of a commercial fishery for a scavenging catfish, the piracatinga Calophysus macro- pterus, for which large numbers of botos were killed for use as bait. This ban (IBAMA, 2014) commenced in 2015 for a 5-year period, and was largely, although not entirely, effec- tive. Given the rate of decline of boto populations, the re- newal of the ban on a permanent basis is crucial; a neces- sary, but not sufficient, measure to address anthropogenic dolphin mortality. Effective control of the use of the most damaging gillnet types will also be needed urgently if boto calves are to survive in sufficient numbers to replace adult losses.
Acknowledgements This study was part of Projeto Boto, a coop- erative agreement between the National Amazon Research Institute (Instituto Nacional de Pesquisas da Amazônia; INPA/MCTIC) and the Mamirauá Sustainable Development Institute (MSDI-OS/MCTIC). We thank the many interns of Projeto Boto who contributed to data collection, and Admisson M. Carvalho, who participated in 90% of the surveys. Funding was provided by INPA/Ministério da Ciência, Tecnologia, Inovações e Comunicações (MCTIC), and Associação dos Amigos do Peixe-boi (AMPA)/Petrobras Socioambiental Program.
Author contributions Study design, fieldwork: ARM, VMFdS; data analysis, writing: ARM.
Conflicts of interest None.
Ethical standards The research abided by the Oryx guidelines on ethical standards, and was approved by the Ethical Standards Committee of the Brazilian Government Institute under which it was carried out (Instituto Nacional de Pesquisas da Amazônia, Manaus).
References
ARAÚJO,C. &WANG,J.(2015) The dammed river dolphins of Brazil: impacts and conservation. Oryx, 49, 17–24.
BRAULIK, G.T., SMITH, B.D. & CHAUDHRY,S.(2012) Platanista gangetica ssp.
minor.In The IUCN Red List of Threatened Species
Oryx, 2022, 56(4), 587–591 © The Author(s), 2021. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605320001350
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